A compound electrode relying on lead is disclosed. The lead can form a base for the electrode. The active surface for the compound electrode may comprise valve metal. The electrode is particularly serviceable in an electrolytic cell used for electrowinning of a metal.
Historically, lead or lead alloy anodes have been widely employed in processes for the electrowinning of metals, such as copper, from sulphate electrolytes. These lead anodes nevertheless have important limitations such as undesirable power consumption and anode erosion. This anode erosion can lead to sludge production and resulting contamination of one or both of the electrolyte and the electrowon product.
During the time that these lead electrodes have been in use, a major breakthrough in anode advancement led to the development of the dimensionally stable anode, principally for use in the chlor-alkali industry. This anode relied typically on a coated valve metal. There then followed attempts to utilize concepts behind this advance so as to devise an improved lead electrode such as for copper electrowinning.
One conceptual approach was to unite in some way the desirable characteristics of a valve metal, e.g., the excellent acid resistance of a valve metal such as titanium, with the desirable features of the conventional lead anodes, including the presence of an oxide that can be an electroconductor. Using this approach, it was proposed to make a composite anode from a sintered article of one metal, e.g., the titanium, which article is infiltrated with the other metal, i.e., the lead. These anodes have been proposed, for example, in U.S. Pat. No. 4,260,470. The titanium can be ground, compressed and sintered to prepare a titanium sponge as a porous matrix. This matrix is then infiltrated with molten lead or lead alloy. The object is first to provide planar anodes in the form of strips. The strips are then joined together in a parallel, co-planar array to provide a large sheet anode.
The patent teaches employing these anodes particularly for use in electrowinning zinc or copper from sulfate electrolytes. However, if the sintered metal is infiltrated with lead, under the anodic conditions that are present such as in a copper electrowinning cell, the lead is anodically oxidized to lead dioxide. Thus, the anode can present loss of lead to the electrolyte, with resultant sludge build-up, and/or require electrolyte additives to deter such loss. Therefore, these anodes are ostensibly better suited for use in lead-acid batteries. Such utility has been disclosed in U.K. Patent Appln. No. 2,009,491A. In any event, there is today no known utilization of these anodes commercially such as in the copper electrowinning industry.
It has also been proposed to retain the commercially acceptable lead anodes, while fully utilizing the technical advance of the coated valve metal development. To this end, ways have been considered as to how to shield the lead from electrolyte, so as to reduce, to eliminate, lead erosion. Thus, it has been proposed to prepare catalytic particles of a metal such as titanium, which particles are activated with a platinum group metal. These particles are then uniformly distributed over, and partly embedded within, the surface of an anode base of lead or lead alloy. The lead plate is thus covered with a layer of these particles, such as of activated titanium sponge particles. Such an anode has been disclosed in U.S. Pat. No. 4,425,217. Therein it is taught that the anode offers improved electrochemical performance for anodically evolving oxygen in an acid electrolyte, and use is taught such as in the electrowinning of metals. However, it was found to be uneconomically viable to scale up this concept and to provide a uniform layer of small particles on the surface of commercial lead electrodes. In working with a multitude of particles, it was further found that the resulting article was difficult to refurbish. As a result, there is no known commercial use today of this anode.
Despite these developments, there then has not yet been found a commercially practicable anode, as a replacement for lead or lead alloy anodes, in industries such as copper electrowinning from sulfate electrolyte. Even today, decades after the development of the dimensionally stable anode for use in the chlor-alkali industry, the anode of choice for copper electrowinning is still the historical lead or lead alloy anode. There is thus a need for an anode, particularly for electrowinning of a metal, which is serviceable for extended stable operation. As an example of this need, even today it is not unusual to remove from 80 to 100 pounds of sludge, comprised principally of lead oxide and lead sulfate, after only about a week of operation, from a single commercial copper electrowinning cell that uses lead anodes. There is not only still the need for a commercially practicable as well as stable anode, but also the need for one which can be readily prepared for reuse and, in reuse, provide similar, extended operation. Therefore, it would be desirable to provide an anode, as either a fresh or refurbished anode structure, having stability, economy of operation, and economy of preparation as a fresh or refurbished structure.
There is now provided a compound electrode, particularly for electrowinning of a metal, which is serviceable not only for extended operation, but which can be readily prepared for reuse and, in reuse, provides similar, extended operation. The compound electrode is provided with either a fresh or refurbished lead electrode segment, having economy of preparation as a fresh, or as a refurbished, item. This lead compound electrode can have desirably low operating voltage and can offer enhanced current density in cell operation. It can serve to minimize or eliminate loss of lead to the electrolyte, which usually proceeds due to electrochemical oxidation as well as erosion of the lead. For convenience, such oxidation plus erosion may more simply be referred to herein as lead xe2x80x9ccorrosionxe2x80x9d. This innovative compound electrode can thus provide for desirable electrolyte cleanliness as well as product cleanliness. The electrode can provide for further operating economy such as by reducing to eliminating the need for electrolyte additives, e.g., the elimination of the use of cobalt addition in a copper electrowinning bath. The compound electrode can not only be easy to assemble as a fresh electrode, but also can be spot repaired and, in refurbishing, such may be done by field installation.
In a first aspect, the invention is directed to a compound electrode for electrowinning a metal present in an electrolyte in an electrolytic cell by partially submersing the electrode in the cell electrolyte, such electrode comprising a thin and solid lead electrode base and at least one thin valve metal surface member in mesh form, which lead base is in sheet form and has broad, essentially rectangular front and back surfaces as well as narrow side and bottom surfaces, with each front and back surface having at least substantially parallel side edges, as well as having top and bottom edges, with the electrode comprising exposed lead side surfaces as well as exposed front and back surface portions above an electrolyte-air interface of the cell, and which metal mesh surface member has a multitude of voids exposing the lead base underlying these voids, with the valve metal mesh surface member extending at least substantially from side edge to side edge across at least one of the broad front and back surfaces of the base, while extending from below the top edge of the base, but above said electrolyte-air interface of the cell, to at least substantially the bottom edge of the lead base, which valve metal mesh surface member has a front, active major face presenting an electrochemically active surface in mesh form for the compound electrode, and a back major face which faces a broad surface of the lead base, and wherein the mesh surface member is combined with the lead base in electrically conductive contact.
In one aspect, the invention is directed to a compound electrode comprising an electrode base of lead or lead alloy and a valve metal mesh member combined with the lead electrode base, which lead base is in sheet form and has a large broad surface, and which valve metal mesh member has a multitude of voids and is in electrically conductive contact with the lead base, which valve metal mesh member is in sheet form and has a front, coated major face and a back major face, with the back major face of the valve metal mesh member facing the lead base and wherein the mesh member is combined with the lead base in electrical contact, whereby a substantial portion of the lead base broad surface is retained in exposed form by the multitude of mesh member voids, while the valve metal mesh member at the broad surface projects a coated face from the lead base and presents an active surface in mesh form for the compound electrode.
In a related aspect, the invention is directed to a compound electrode as is generally described in the paragraph immediately hereinabove, but having a metal mesh member that may be other than a valve metal mesh member, which member may not be coated and can include a platinum group metal mesh member.
In another related aspect, the invention relates to an electrolytic cell containing a compound electrode as described hereinabove.
In another aspect, the invention relates to the method of providing an electrode assembly, which assembly has a lead base useful in an electrochemical process, such lead base being spaced apart in the cell from a cell electrode, with a gap maintained therebetween for containing an electrolyte, which method comprises:
establishing the lead base with a broad surface whereby the lead base serves as an assembly support structure;
establishing a mesh member with a broad front face and a broad back face;
coating the mesh member front face to provide an active front face;
combining the mesh member with the lead support structure, with the mesh member broad back face facing the broad surface of the lead support structure;
securing the mesh member to the lead support structure in electrically conductive engagement for forming the electrode assembly; and
electrically connecting the lead support structure of the electrode assembly to a power supply, the support structure serving as a current distributor member for the mesh member.
In another aspect, the invention pertains to an apparatus for electrodepositing a metal from an electrolyte, the apparatus having a cathode, an anode spaced from the cathode providing a gap containing the electrolyte therein, wherein an apparatus electrode has an active electrode member plus a support structure, the apparatus comprising:
a stationary and rigid lead support structure for the apparatus electrode, which lead support structure has a broad surface;
a flexible mesh member for the apparatus electrode, which mesh member has a broad, coated front face and broad back face, with the broad back face of the mesh member facing the broad surface of the lead support structure;
means securing the mesh member to the lead support structure in electrically conductive engagement, the securing means providing inflexible positioning of the mesh member in relation to the lead support structure; and
power supply means providing electrical power to the lead support structure whereby the lead support structure serves as an electrically conductive current distributor member for the mesh member.
Previously, where it has been desired to use the lead as an electrode base, it has been the practice to cover the operative surface of the lead base. This may be accomplished with a layer of uniformly distributed particles. It can also be done with a sheet anode which may be in strip form. These arrangements have been discussed hereinabove. It has now, however, been found that such full coverage of the lead base is not necessary. The compound electrode structure of the present invention with an open mesh member can achieve desirable electrolytic activity, and achieve this activity without deleterious lead contamination in the electrolyte. This can be obtained even for the compound electrodes wherein the mesh members, with their inherent void fraction, leave exposed a very substantial portion of the lead base. As representative, where mesh members leave exposed on the order of about 50 percent of the lead base surface area over which the mesh extends, nevertheless lead contamination in cell electrolyte may be reduced by as much as 95 percent, or even more, by the innovative compound electrode.
Furthermore, as is the case for lead anodes utilized in copper electrowinning, the compound electrode can be completely retrofittable. No redesign of cell electrical systems may be needed and existing buss connections can be retained. Also, for cell electrolytes, it can be possible to maintain existing compositions and flow rates, although it is contemplated that less expensive compositions may be achieved through the reduction to elimination of electrolyte additive.